Chapter 72 Surgical Treatment of Paraclinoid Aneurysms

The portion of the proximal intradural internal carotid artery (ICA) adjacent to the anterior clinoid process (ACP) is called the paraclinoid segment. Aneurysms arising from the ICA between the roof of the cavernous sinus and the origin of the posterior communicating artery (PComA) are defined as paraclinoid aneurysms.¹^–⁵ These aneurysms are of considerable surgical interest due to their particular anatomic features and technical difficulties. Some of these aneurysms were considered in the past as unclippable or associated with very bad results when surgically approached.⁶ Fortunately, with the progressive refinement of microsurgical techniques their management has changed from the very conservative surgery to direct neck clipping, with results surpassing those of the endovascular therapy in terms of total neck obliteration and long-term recanalization (Table 72-1).^4,^5,⁷ The classification of these aneurysms according to the origin of their necks and projection of the aneurysms is particularly important to select the optimum microsurgical approach.^1,⁸^–¹²

Table 72-1 Comparative Results in Surgical Series of Paraclinoid Aneurysms

Incidence

Reported incidence of paraclinoid aneurysms varies from 5% to 9% of anterior circulation aneurysms. They are more frequent in female sex and tend to occur in association with multiple aneurysms in more than 20% of the cases, including the so-called mirror aneurysms. They tend to increase their size to become large or giant, presenting in these cases with visual deficit rather than subarachnoid hemorrhage.

Anatomic Aspects

In 1938, Fisher ^12a published an anatomic nomenclature for the ICA based on the angiographic course of the artery, describing five segments designated C1 through C5. However, these segments were numbered opposite to the direction of blood flow and the extracranial ICA was excluded (Fig. 72-1A). Recently, other classifications have been published that include the extracranial and intracranial segments, and the carotid segments have been numbered according to the direction of the blood flow. Therefore, the paraclinoid segment of the ICA comprises the C2 and C3 segments of the original Fisher classification (1938), the distal C3 and Proximal C4 segment of Gibo et al. (1981),¹³ and the C5 and C6 segment of the Bouthillier classification (1996)¹⁴ (Fig. 72-1). Because of the close topographical vicinity of these aneurysms to osseous, fibrous, nervous and vascular structures of the skull base they may present with clinical symptoms due to compression of the optic nerve or other surrounding structures instead of the classic subarachnoid hemorrhage frequently seen in aneurysms in other locations.^3,¹⁵ The anatomic structures of the paraclinoid area not only produce a limited space for expanding vascular lesions, but also for the neurosurgeon during operation, thus, sufficient proximal control and minimal manipulation of the vascular and nerve structures around are of utmost importance for the postoperative outcome.^1,^2,^4,^5,^7,¹⁶^–¹⁸

FIGURE 72-1 A, Classification of the internal carotid artery segments by Fisher (1938). B, Classification by Gibo et al. (From Gibo H, Lenkey C, Rhoton AL Jr. Microsurgical anatomy of the supraclinoid portion of the internal carotid artery. J Neurosurg. 1981;55:560-574.) C, Classification by Bouthillier et al.

(From Bouthillier A, van Loveren HR, Keller JT. Segments of the internal carotid artery: a new classification. Neurosurgery. 1996;38(3):425-433.)

Classification

Given the variability in projection, size and origin of these aneurysms, various authors in the past have classified them. Most of the classifications are based on the site of origin of the neck, the projection of the dome and its relationship with branches arising from the ICA.^2,⁷^–¹⁰ ^,12 ^,17 The vast majority of saccular aneurysms arise within the angle formed by the parent artery and a significant arterial branch. Therefore, aneurysms related to these arteries are called accordingly, for example, ophthalmic and superior hypophyseal artery aneurysms.^2,⁸ In addition, aneurysms unrelated to branches occur only rarely in this segment (distal ophthalmic aneurysms).¹² Ventral paraclinoid carotid aneurysms seem to belong to the same category as infraophthalmic aneurysms, which originate from the ventral surface of the ICA and in which the proximal aspect of the neck is located approximately at the level of the ophthalmic artery and the distal aspect of the neck is located proximal to the posterior communicating artery¹⁹ (Fig. 72-2). These aneurysms also project straight or slightly medial and downward. Carotid cave aneurysms are another distinct type of aneurysm located at non-branching sites of this segment. This type of aneurysm arises from the medial wall of the proximal intradural ICA, and grows within a small dural recess, with the apex of the sac directed toward the cavernous sinus¹⁰ (Fig. 72-3). All these denominations have contributed to create a rather complex view of these aneurysms. Another confusing characteristic is that they are not always related to a branching artery and may point in any direction, as laterally, medially, ventrally, or dorsally (Fig. 72-2).

FIGURE 72-2 Projection and growth of paraclinoid aneurysms. A, Dorsal type aneurysms. Proximal type (ophthalmic) and distal dorsal type arise from the dorsal surface of the ICA between the ophthalmic artery and the ICA bifurcation. Growing produce a direct compression of the optic nerve causing visual acuity and field defects. B, Ventral type aneurysm. This aneurysm originates in the ventral or ventromedial surface of the ICA. When growing, they produce an upper displacement of the ICA with less optic compression than the dorsal type. They tend to occupy the space medial or lateral to the ICA, causing compression of surrounding structures (e.g., cranial nerve III). C, Paraclinoid aneurysms may increase in size according to the space around the site of origin. They may grow upward, medially, ventrally, laterally, or in a combination of these directions.

FIGURE 72-3 A to F, Carotid cave aneurysms. This aneurysm arises as transitional type of aneurysm between cavernous sinus and intradural paraclinoid. Located between proximal and distal carotid rings with ventromedial direction, they are found incidentally or because of subarachnoid hemorrhage. Most are <15 mm when discovered. Surgical approach should include opening of distal dural ring and packing cavernous sinus to avoid venous bleeding. Patient in panel F had multiple aneurysms (arrows) located at left middle cerebral artery, posterior communicating segment of the ICA, and carotid cave.

In order to simplify the classification of these aneurysms, we prefer to name them according to their site of origin in relation to the circumference of the ICA and some branching artery (if any), because this is also relevant at the time of clipping. Thus, the paraclinoid aneurysms can be classified as follows: dorsal type aneurysms, ventral type aneurysms, carotid cave aneurysms, and global type aneurysms (Figs. 72-3 to 72-7).

FIGURE 72-4 A to F, Proximal dorsal type aneurysms. Also defined as carotid-ophthalmic aneurysms, they grow at the dorsal surface of the ICA at close proximity with ophthalmic artery. They may reach giant size (>25 mm) before causing neurologic symptoms. The most common manifestation is visual deficit. Surgical technique include the anterior clinoid process resection to identify and preserve the ophthalmic artery. C, Ventral displacement of the ICA can be appreciated (arrow). D, This aneurysm had been coiled before, and showed recurrence in the 6 months following the endovascular procedure, suggesting heavy hemodynamic stress forces in this area.

FIGURE 72-5 A to F, Distal dorsal type aneurysms. This type of aneurysm origin at the dorsal surface of the ICA, away from the origin of the ophthalmic artery (arrows). They usually do not progress to large or giant sizes, and present with subarachnoid hemorrhage as initial symptom. This is also a frequent site of location of the so-called blood blister–like aneurysms. In these cases, the resection of the anterior clinoid process may not be necessary to get proximal vascular control and expose the aneurysm. However, in case of blood blister–like aneurysms, special designed clips to encircle the ICA or a wrapping procedure may be necessary to treat these aneurysms. F, This patient had two dorsal type aneurysms, one proximal type (ophthalmic) (arrowhead) and one distal type (arrow).

FIGURE 72-6 A to F, Ventral type aneurysms. Ventral type aneurysms originate opposite ophthalmic artery at ventral surface of ICA. They grow between distal dural ring and origin of posterior communicating artery and produce upper displacement of ICA. According to available anatomic space, they may grow medially or laterally. The most effective way to clip these aneurysms is using fenestrated clips.

FIGURE 72-7 A to D, Global type aneurysms. This type of aneurysm represents technical challenge for reconstruction because site of origin cannot be well-defined as dorsal or ventral. In these cases, it is better to select a surgical technique based on proximal occlusion of the ICA if good collateral flow exists, or to perform bypass procedure to ensure blood flow distal to aneurysm.

Dorsal Type Aneurysms

These include the proximal dorsal type aneurysms that correspond to the carotid ophthalmic aneurysms. They arise from the ophthalmic segment of the ICA in close relationship with the ophthalmic artery.^7,^9,^11,^15,¹⁸ On the lateral view of an angiogram the neck of the aneurysm is located just distal to the origin of this artery (Figs. 72-2A, 72-4, and 72-8). They grow upwards and cause compression of the optic nerve. As they have a tendency to grow, many of these lesions are detected because of visual deficits. The second main type is the distal dorsal type aneurysm (also known as dorsal wall aneurysms). This aneurysm grows upward at the dorsal surface of the ICA (Fig. 72-5). They are located distal to the ophthalmic artery and seem not to origin from any branch of the ICA.^9,¹² Whether they arise at bifurcations of vestigial arteries or because hemodynamic stress at the curvature of the carotid siphon is unknown.²⁰ The dorsal surface of the ICA is also a common site of blood blister–like aneurysms. These are dangerous small lesions with fragile walls consisting of normal adventitia or fibrin nets. Primary treatment in the acute stage is challenging due to the substantial risk of intraoperative bleeding, resulting in the formation of a large defect in the ICA. To treat these aneurysms, sometimes it is necessary to use especially designed encircling clips or some wrapping procedure, otherwise, a bypass procedure with trapping of the aneurysm can be used as an alternative.

FIGURE 72-8 Demonstrative cases of dorsal type aneurysms. A and B, Pre- and post-operative angiograms show large aneurysm with medial extension. Postoperative image shows the clip position. Two straight 15-mm clips were used to obliterate the aneurysm. C and D, This aneurysm had a rather straight upward direction with a small neck. However, during surgery, it was noted that the aneurysm wall was thick. The neck was closed using a 11-mm straight clip and a miniclip for a small neck remnant. E and F, Giant aneurysm of dorsal type. The patient is a 40-year-old female admitted to the hospital because of visual deficit. At surgery we found a thin-walled aneurysm. The aneurysm could be obliterated using two 11-mm straight clips.

Ventral Type Aneurysms

These aneurysms grow at the ventral or ventromedial surface of the ICA (Figs. 72-2B, 72-6, and 72-9). They are located opposite to the origin of the ophthalmic artery and in close relationship with the superior hypophyseal artery. As they increase in size, they are directed downward and medially. When large or giant, they produce an upward displacement of the ICA; however, visual disturbance is not as frequent as in dorsal type aneurysms.^1,^2,^5,^8,¹⁹

FIGURE 72-9 Demonstrative case of ventral type aneurysm. A, Even when aneurysm could be confused with dorsal type, at initial filling of sac, the course of ICA is visible over aneurysm dome (arrow). B, After sac of aneurysm is filled with contrast product, site of origin cannot be visualized. With this information, we expected to find aneurysm dome ventral to ICA. C, Postoperative anterior view shows total obliteration of aneurysm preserving course of ICA. D, Three-dimensional oblique view shows clip arrangement in tandem fashion to reconstruct ICA using ring clips.

Carotid Cave Aneurysms

This is a special type of aneurysm originating between the proximal and distal carotid rings. They grow ventromedially proximal to the ophthalmic artery, and are visible mainly on the anterior or oblique angiographic views. On the lateral view, they remain hidden by the ICA (Figs. 72-3 and 72-10). Carotid cave aneurysms are transitional in type between paraclinoid intradural and cavernous sinus aneurysms. They may grow out of the cave into the intradural subarachnoid space. During surgery, they project ventrally at the level of the carotid genu ¹⁰ (Fig. 72-11). In this sense, the surgical genu is located more proximal than the angiographic genu, which roughly corresponds to the location of the dural ring.

FIGURE 72-10 Carotid cave aneurysm, demonstrative case. This 32-year-old female patient arrived at the hospital with Hunt and Kosnik grade 2 subarachnoid hemorrhage. Angiogram showed a carotid cave aneurysm. A, Anterior view. Aneurysm is visible with medial and ventral directions. B, Oblique view shows the aneurysm projecting under genu of the ICA. C, In lateral view, aneurysm is usually hidden by genu of ICA (arrow). This is a very particular characteristic of small carotid cave aneurysms. D, Postoperative lateral view shows position of ring clip used to clip the aneurysm. Three-dimensional reconstruction image in the anterior and lateral views (E and F) shows clip position and elimination of the aneurysm.

FIGURE 72-11 Surgical approach to carotid cave aneurysms. Drawing shows completed surgical steps to reach carotid cave region. At first clinoid region was exposed through sylvian fissure opening and dissection of basal cisterns around ICA and optic nerve. The anterior clinoid process has been drilled and optic nerve was unroofed. Dura along course of optic nerve has been opened including falciform ligament. Cavernous sinus has been packed with fibrin glue and genu and axilla of ICA have been exposed. At this time, ICA can be laterally mobilized to expose neck of aneurysm. Clipping using a fenestrated clip can then be completed.

Global Type Aneurysms

These aneurysms involve the entire circumference of the ICA; they are large or giant in size, and during angiography or surgery the origins of the neck are not as easy to identify as ventral or dorsal types even when the origin was surely at any of these points. Most cases are associated with degeneration of the carotid wall. The importance of this type of aneurysm is that the treatment is based on deconstructive techniques (parent artery obliteration and bypass surgery). This type of aneurysm should not be diagnosed only because of size and shape (Fig. 72-7).

It should be emphasized that during the growth process of a paraclinoid aneurysm, the dome could occupy anatomic spaces at the medial or lateral side of the ICA (Fig. 72-2). We do not believe that the medial or lateral sides of the ICA are origin sites of such aneurysms. When we carefully analyze the angiographic videos and films or the intraoperative recordings (except for global type aneurysms), the origin can be traced to the dorsal or ventromedial surface of the ICA.